This invention relates to extrusion dies and more particularly to an extrusion die for producing a lined extrudate or a compound extrusion of two materials.
The extrusion of materials, particularly thermoplastic materials, is a highly developed art. Dies are available, for example, for producing not only rods and tubes but special shapes having almost any type of cross section. In addition, shapes having inlays, overlays and striations may be produced. Most of the available dies, however, have been designed for extrudates that are of chemically and physically compatible materials and where two materials have been so simultaneously extruded the purpose has been to obtain the other incidental benefits such as different color or the different resistance to the environment that the two materials may exhibit. Thus, "candy-striped" tubing, color-coded insulation sleeves for electrical wires and tubing that is both gasoline and water-resistant have been produced heretofore.
With respect to the extrusion of materials having different melting points, dies are known such as that disclosed in M. J. Drabb U.S. Pat. No. 3,538,547 issued Nov. 10, 1970, which shows a wire die for simultaneously extruding a coating of polyvinylchloride (PVC) having a melting point of 380.degree.-390.degree. F., but which burns at 410.degree. F., and an overjacket of nylon having a melting point of 450.degree.-500.degree. F. The Drabb patent die is able to achieve co-extrusion of these materials requiring such divergent temperatures by thermally insulating the respective extrusion orifices from each other within the die head so that each may operate at its proper temperature. On the other hand, Dimitroff U.S. Pat. No. 3,640,659 issued Feb. 8, 1972, purports to overcome the temperature difference problem of the two extrudates (poly-ethylene having a melting temperature of 210.degree.-225.degree. C. and polyvinylidine chloride ("SARAN") having a melting temperature of 130.degree.-140.degree. C.) by merging the two extrudate streams after they exit the die orifices.
Techniques are also known for attempting to cope with the problem of bonding dissimilar resins. For example, an intermediate resin compounded of the two dissimilar materials may be extruded to form a three-layer sandwich as disclosed in Maillard et al U.S. Pat. No. 3,561,493. Another technique is shown in L. Klenk et al U.S. Pat. No. 3,486,196 which suggests that materials extruded in the form of concentric tubes may be bonded after emerging from the die orifice by stretching one of the extrudates with respect to the other or by what is referred to therein as "internal calibrating".
Recently, the public has become concerned with the level of certain chemicals in the environment. Vinyl chloride has been identified in the public mind as a particularly hazardous chemical. In the food and beverage industry, which makes extensive use of PVC tubing and piping, the concern has been voiced whether this plastic material itself may exhibit some of the dangers of vinyl chloride or whether it may under various ambient conditions or exposures exude free vinyl chloride even to the slightest degree. In addition to the vinyl chloride which is used in fabricating the basic polyvinylchloride resin used in the extrusion of the PVC tubing, plasticizers such as benzyldiethylhexylpthylate and stabilizers such as soy bean oil or epoxy are used. Depending upon the manufacturing process, some of these ingredients may not be fully incorporated and under extreme conditions there is concern that some of these ingredients may leach out into the food or liquid stream carried by the PVC piping. Although the level of free vinyl chloride that may exist in a finished article of PVC may be undetectable without the use of sensitive instruments, the public concern has nevertheless led some manufacturers of food dispensing and vending machines to search for materials to substitute for PVC.
One of the substitute materials that has been tried is ethylene vinyl acetate (EVA). Tubing made of EVA contains no vinyl chloride, and usually no plasticizer or stabilizer is necessary. Accordingly, EVA tubing appears to have no materials in its make-up that may be the subject of suspicion from the health safety viewpoint. Unfortunately, EVA tubing does not have the same resistance to discoloration and abrasion in handling as does PVC tubing. This characteristic is particularly troublesome in the vending machine industry because many local health department inspectors charged with examining food vending machines have adopted a priori rules according to which a machine may be given an unsatisfactory rating when its food or potable liquid conducting tubings appear to be "dirty". As a result, EVA tubing which is basically clean and sanitary and thoroughly serviceable may nevertheless be the cause for a local health department inspector condemning the machine because of the tubing's external appearance. Accordingly, the food handling industry has generated a need for a clear plastic tubing that has good mechanical properties, resistance to abrasion, grease, and dirt collection and which cannot leach or exude undesirable chemicals into the food or other liquids carried therethrough.
It was initially thought that tubing with the desirable physical characteristics of PVC and none of its suspected health hazards might be fabricated by a co-extrusion of PVC and EVA with the EVA forming an internal lining to the PVC such that the food or other liquid stream carried by the tubing could be exposed only to the EVA and could not come into contact with the PVC. Such a material would have PVC on the outside to resist abrasion and discoloration or "dirtying" by routine handling and EVA on the inside for its presumably safer characteristics from the health standpoint. However, the fabrication of such a compound tubing posed a great problem because of the difficulty of achieving good bonding between the EVA and the PVC. Tubing in which the EVA and PVC walls are not firmly bonded may appear to exhibit cloudy areas where there is poor contact between the materials and such cloudy or bubbly tubing will not be acceptable.
Additional problems are encountered in attempting to produce a compound tubing where the EVA forms only a thin-walled lining on the interior of a heavier-walled PVC hose. If it is attempted to extrude the PVC externally to an upstream-extruded EVA tube, the different required material flow rates preclude economical operation. If conventional dies are used, the bonding problem remains to be solved. Accordingly, the need has developed for a die capable of extruding EVA lined PVC tubing at useful production rates and which can assure good bonding of the two materials.